The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses (“PALs”) are used for the treatment of presbyopia. PALs have at least one progressive surface that provides far, intermediate, and near vision in a gradual, continuous progression of vertically increasing dioptric power from far to near focus, or top to bottom, of the lens.
PALs are appealing to the wearer because PALs are free of the visible ledges between the zones of differing dioptric power that are found in other multifocal lenses, such as bifocals and trifocals. However, an inherent disadvantage of PALs is the presence of unwanted astigmatism, or astigmatism that is undesirable and is introduced or caused by one or more of the lens' surfaces. Unwanted astigmatism can produce swim, image magnification, and motion effects for the lens wearer that disrupts the wearer's vision. In hard PAL lens designs, the unwanted astigmatism borders the lens channel and near vision zone. In soft designs, the unwanted astigmatism may extend into the distance zone.
Typical PALs have a 12 to 14 mm in length corridor between the fitting point and the point along the prime meridian of the lens at which the power reaches 85% of the lens' add power. Although such a corridor helps provide more gradual power changes and lower levels of unwanted astigmatism, the near viewing zone is often cut-off when lenses of these designs are mounted into standard frames.
Some PAL lenses have been designed with short corridors in the 9 mm to 12 mm range, but these PALs typically have much higher levels of unwanted astigmatism. If a PAL wearer has become accustomed to a particular design in a standard size frame and then chooses to switch to a smaller frame, typically the ECP must switch the patient into another PAL brand, which may result in wearer adaptation problems.